Technique for monitoring galvo angle

ABSTRACT

Disclosed is a radiant energy optoelectronic position monitor system which detects the angular position of a &#34;galvonometer mirror&#34; (beam scanner) used in an optical disk memory arrangement.

BACKGROUND OF THE INVENTION

The present invention relates generally to methods for opticallyrecording and/or reading data, on disks and more particularly to improvemethods for selecting and following data tracks thereon.

In recent years considerable effort has been expended to developimproved methods and apparatus for optically recording and reading on asuitable medium because of the unusually high recording densitypotential offered by optical recording. Examples of various knownmethods and approaches are revealed in the following references:

    ______________________________________                                        Patent No.   Date Issued  Inventor(s)                                         ______________________________________                                        4,216,501    8/5/80       Bell                                                4,222,071    9/9/80       Bell, et al                                         4,232,337    12/4/80      Winslow, et al                                      4,243,848    1/6/81       Utsumi                                              4,243,850    1/6/81       Edwards                                             4,253,019    2/24/81      Opheij                                              4,253,734    3/3/81       Komurasaki                                          4,268,745    5/19/81      Okano                                               ______________________________________                                    

Publications

R. A. Bartolini, et al., "Optical Disk Systems Emerge", IEEE Spectrum,August 1978, pp. 20-28. G. C. Kenney, et al., "An Optical Disk Replaces25 Mag Tapes", IEEE Spectrum, February 1979, pp. 33-38. K. Bulthuis, etal., "Ten Billion Bits on a Disk", IEEE Spectrum, August 1979, pp.26-33. R. Michael Madden, "Silicon Position Sensing Detectors forPrecision Measurement and Control", SPIE, Vol 153, Advances in OpticalMetrology (1978). Robert M. White, "Disk-Storage Technology", ScientificAmerican, 243: 138-148 (August, 1980).

The subject matter of these references is to be considered asincorporated herein to the extent relevant.

SUMMARY OF THE PRESENT INVENTION

The primary purpose of the present invention is to provide significantlyimproved methods and apparatus over those disclosed in the foregoingreferences for optically recording and/or reading data from an opticalstorage medium.

One disk memory storage technique uses a system of lenses and mirrors tofocus a laser beam onto a rotating disk D coated with a thin layer ofmetal as shown in FIG. 1. Data is recorded on the disk by formingconcentric of microscopic holes in the metallic layer with a powerful,focused beam; data is read back by passing a less powerful beam over thedata tracks and detecting the intensity changes in the reflected light.This technique allows data densities many times greater than magneticdisk memories, but the microscopic nature of the storage medium requiresa correspondingly precise method of positioning the focused laser beams.

The final beam-positioning element in the lens and mirror system is agalvanometer, or "galvo", G, an electromagnetically pivoted mirror g_(m)that scans the laser beam radially on the disk [while the entire galvounit G is to be reciprocated across disk-tracks for "coarse seek" oftrack location--the tilting of mirror g_(m) providing "fine-seek"]. Forclosed-loop operation of the "beam position control system", it isnecessary to communicate to the control system a feedback signalindicating the angular position of the galvo mirror. Such a controlsystem should enhance the accuracy and response time of the beamposition; also the effects of cross-coupling from nearby linear motorsand other disturrbance sources should be minimized. This invention isintended to teach such an improved control system.

One objective hereof is to provide better "control feedback", i.e., toteach the use of a position sensor to determine the angle a galvo mirroris pivoted [relative to the chassis on which it is mounted].

Such a position detector is preferably operated with an infra-red beamreflected by the galvo mirror through a simple lens arrangement todetect the mirror's angular position. This optical approach isadvantageous over magnetic or capacitive transducers in that (a) it isimmune, in principle, to electrostatic and magnetic interference causedby the focus motor and (b) the infra-red optical system can bedistinguished from the laser beam while also made an analog of thelaser's optical system, wherein galvo mirror rotation results in alateral shift of a focused spot on a flat surface.

A "folded" optical system with a fixed, secondary mirror can allow thegalvo mirror to be located optimally close to the focus motor and itscontained lens system, make the assembly compact, and avoid blocking thelaser beam.

To provide shielding from ambient light sources and from magneticfields, the assembly can be enclosed in a mu-metal box.

In a particular preferred embodiment of the present invention, thereliability and accuracy of optical recording and reading with respectto a rotating optical disk is significantly enhanced by the employmentof such a position-sensor unit which functions in conjunction with athree-beam laser arrangement and read signal processing electroniccircuitry so as to provide significantly improved and more accuratecontrol over recording and reading operations.

The specific nature of the invention as well as other objects,advantages, features and uses of the invention will become evident fromthe following description of a preferred embodiment taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram showing an optical disk memoryarrangement apt for using the invention;

FIGS. 2 and 3 show a preferred embodiment in plan and side simplifiedview respectively; while

FIG. 4A is a simplified tracing of the related "monitor rays" and FIG.4B shows the same for an "unfolded" array;

FIG. 6 is a very simplified side view of the preferred detector for theembodiment of FIGS. 2-4, with an enlarged plan view thereof in FIG. 5and electrical circuit model thereof in FIG. 7;

FIG. 8 is a block diagram of a preferred utilization system apt for usewith the embodiment above, while FIGS. 9, 9A, 9B, 10, 10A-10F arepreferred circuit elaborations thereof;

FIG. 11 is a schematic of a test array for testing linearity of such anembodiment;

FIG. 12 is a modification of the FIG. 2 embodiment in similar view,while FIG. 13 is a simplistic view of a further modification thereof;

FIG. 14 is an overall block diagram of an optical recording andreproducing system incorporating a preferred embodiment of the presentinvention;

FIG. 15 illustrates the relative locations of the three laser beamsprovided by the system of FIG. 14 when focused on a selected track ofthe optical disk;

FIG. 16 is a block, schematic representation of the laser optical systemshown in FIG. 1;

FIG. 17 is a schematic diagram generally illustrating the arrangementand formatting of data on the optical disk;

FIG. 18 is a schematic diagram illustrating details of the "header"formatting shown in FIG. 17;

FIG. 19 is a block electrical diagram illustrating a preferredimplementation for signal processing electronics apt for use in theforegoing; and

FIG. 20 is a cross-sectional view illustrating the construction of anoptical disk apt for employment in the system of FIG. 1.

DETAILED DESCRIPTION OF EMBODIMENTS

FIGS. 2 and 3 show a preferred embodiment of a position sensor unit GUadapted to determine the angular orientation of a galvo mirror g_(m) asit is pivoted relative to an associated galvo chassis ch and to providea correlative feedback signal (s_(g)) to related means for controllingmirror position. This arrangement will be understood as especiallyadapted for operational association with certain Optical Disk Memory(CDM) units (e.g., like that described below and shown in FIGS. 14-20).The galvo mirror g_(m) will be understood as accommodating beam scanningradially across the disk for track selection/centering (e.g.,"track-seek", "track-follow" in conjunction with translator means tmFIG. 1) as known in the art, these are typically controlled tocompensate for variances in disk position (runout correction).

And, except as otherwise specified, all materials, methods and devicesand apparatus herein will be understood as implemented by knownexpedients according to present good practice.

Thus, unit GU will be understood as providing mirror g_(m) positioned toreceive and redirect laser beams (e.g., about 1-2 mm diameter) along aprescribed path (see path LB--LB through apertures ap, ap' in chassisch, FIG. 2), being pivoted by conventional galvanometer means 10,including a coil means 13 (e.g., a pair of 150 gauss coils mounted withmirror g_(m) (e.g. 10×13 mm) on a frame (preferably plastic with amu-metal and copper shield 11) to make up galvo unit 10.

To provide shielding from ambient light and from stray magnetic fields(e.g., produced by nearby focus motor), the assembly is enclosed in amu-metal box, 2 within which a baseplate 1 is affixed to mount unit 10and the other components.

Galvo mirror g_(m) has a mechanical neutral position which it assumeswhen no current flows in the coil. The plastic chassis of the galvo isprovided with a locating lug, whose axis is approximately collinear withthat of the galvo mirror. During alignment, the galvo is pivoted aboutthis lug so that the laser beam is centered in the objective when thegalvo coil is unenergized. This minimizes the current and heat in thegalvo coil during normal operation.

Typically mirror g_(m) is designed as a "good" reflector of the laserradiation (e.g., typically at 6330 A°)--and accordingly should beexpected to be a relatively "poor" reflector of infra-red (e.g., the9200 A° specified below would be only ˜25% reflected, necessitating arelatively high-intensity IR source). Otherwise adjustments may be madeto improve IR reflectance.

Now, workers will appreciate that there are various known, conventionalways to monitor the position of galvo mirror g_(m). One would be toattach magnetic flux means to one or several points along the mirror anddetect positional-shift thereof with associated fixedly-disposed "HallEffect Sensors". A related method would be to dispose capacitor plateson mirror g_(m) and confronting fixed plates coupled to electronic meansto detect mirror movement (inter-plate gap change) as a function ofchanges in inter-plate capacitance. Another distantly relatedarrangement is the Reticon optical detector used with television cameraswhereby a multi-segment (256 unit) monolithic chip monitors theposition-changes of an image moving across the segments and, using clockmeans, shifts-out an image-produced signal as serial data much as with astandard "shift register". This is very, very expensive however.

IR position-monitor:

According to a feature hereof, unit GU is also provided with optical(reflector) means for monitoring the angular position of mirror g_(m)preferably in the form of an IR source, a fixed reflector, an associatedIR detector and intermediate focus means. [Note IR emitter 41, reflector21, IR detector 31 with filter 31-F and intermediate focusing lens 23 inFIG. 2].

Thus, the position detector can use the infra-red beam directed by itsreflector 21 to be reflected by the galvo mirror g_(m) through a simplelens arrangement 23 to detect the angular position of g_(m). Thisoptical approach is advantageous in it is immune (in principle) toelectrostatic and magnetic interference which usually abound in theregion of the focus motor; also the optical system can be made an analogof the laser's optical system wherein galvo mirror rotation results in alateral shift of a focused spot on a flat surface.

A "folded" optical system with fixed IR reflector 21 (see FIG. 4A)allows the galvo mirror to be located close to the focus motor and finallaser objective, while keeping the assembly compact and avoidingblocking of the laser beam.

And preferably, reflector 21 can be adjustably pivoted (duringcalibration) to adjust alignment of the infra-red beam. This adjustmentestablishes the mechanical angular offset of the position detector. Withthe laser beam aimed as described above (galvo coil-current being"zero"), the reflector is adjusted for a circuit signal output (S_(g))of zero. (FIG. 8). When properly adjusted, infra-red light emanates fromthe infra-red emitting diode (IRED), through the plano-convex lens, tothe reflector, to the galvo mirror, and then (symmetrically) backthrough the reflector and lens to form an image of the IRED on thecenter of the detector. The IRED and detector surfaces are placed in thefocal plane of the lens so that light emerging from the other side iscollimated in parallel rays. Thus, the total distance between lens andgalvo mirror will not substantially affect the image location. However,any pivoting of the galvo mirror changes the wavefront relationships ofthe collimated beam and results in the image being shifted laterally inthe direction of beam deflection, (see FIG. 5).

FIG. 4B shows this array (IRED 41, lens 23, reflector 21, mirror g_(m)and detector 31) in an "unfolded" state to facilitate ray tracing. Theinfra-red source 41 is placed in the focal plane of the lens 23 so thatlight emerges through the other side of the lens in collimated rays.When the collimated light re-enters the lens after being reflected bythe mirror system, it converges at the focal plane to form an image ofthe light source there, at detector 31.

The result of galvo mirror deflection (angle θ) is that the image isshifted laterally (distance d) in the direction of beam deflection.Thus, one may describe this as d=f2θ, where f is the focal length of thelens, detector 31 is thus intended to detect shift of mirror angle asconverted to linear position-displacement.

One preferred detector element 31 is a "lateral cell" position detectoras further described (e.g., see description in publication by R. M.Madden, cited above). "Lateral cells" are available (e.g., in single ordual-axis configuration. Lateral cells have one, continuous, extendedactive area. The difference between a lateral cell and otherphotodetectors is that signal currents do not flow through the siliconchip to be collected at the back side; rather they flow laterally untilcollected at ohmic contacts located around the periphery of the activearea.

When a spot of light illuminates a region of the active area of alateral cell, a small local forward bias is induced at the illuminatedregion of the junction. This forward bias causes ohmic currents to flowto each of the collecting contacts (cathodes). The fraction of the totalsignal current collected at the jth contact is given by:

    i.sub.j /j.sub.total =Y.sub.j /(Σ.sub.j Y.sub.j)     (Eq.1)

j=1,2 (single axis

j=1,2,3,4 (dual axis)

Y_(j) in Eq. 1 represents the conductance from the point of illuminationto the jth cathode. It is simply the inverse of the silicon substrateresistance between these points. Since the substrate resistance betweenthe spot and a given cathode is (in the one dimensional approximation)inversely proportional to the distance between the spot and thatcathode, expressions for normalized transfer functions in each axis are:

    .sub.x =Δ.sub.x /Σ.sub.x ; .sub.y =Δ.sub.y /Σ.sub.y (Eq. 2)

In Eq. 2, Δ_(x),y represents the difference between currents collectedby cathodes lying in the x (y) axis. Σ_(x),y represents the sum of thecurrents collected by the cathodes lying in the x (y) axis.

Lateral cells come close to the ideal linear characteristic. This isespecially true of the single-axis device. It appears that nearly alldeviations from linearity observed in lateral cells are associated withthe failure of a one dimensional geometrical model in relating currentsignal to position. Their dynamic operating range is independent of spotsize (excluding edge obscuration effects). Linearity and absolutemeasurement accuracy is also independent of spot size and uniformitycharacteristics to first order. Other advantages include excellentlinearity, large operating range, and electronically adjustable null.

The cell's output signal is directly proportional to the location of the"optical centroid" (equivalent to the averaged intensity-center here),of the light pattern falling on the lateral cell's active silicon area.Since the outputs react to the "centroid" of the light, the imagefalling on the detector need not be sharply focused or of uniformintensity, as long as it stays consistent as the beam is scanned.

FIG. 5 illustrates the effect of lens focal length on image position.For example with a 25 mm lens f/2 23 used (to reduce system size and toutilize the center of the active area, which should have betterlinearity than the extremes) an image about 1.85 mm diameter could shiftabout 1.3 mm (θ=±1.5°) across the lateral cell face 31-5 (here assumeactive area: 2×5 mm). Note that as more room is available for opticalsystem, a larger area and longer lenses will be feasible.

A preferred source 41 is a 12 mW LED (Litronix model LD 271) with adiffuser type lens and no virtual image. This gives (with lens 23) animage 1.85 mm (diameter) falling on the detector's active area. Toovercome light losses in the optical system, a powerful infra-redemitting diode is chosen. This Litronix lateral cell (LD 271) is a 12milliwatt device having a diffuser lens which proved ideal for thisapplication. IRED's having clear (built-in) lenses were rejected becausetheir lenses create a virtual image of the light source behind the IREDpackage (also enlarged).

The length of cell 31 should, of course, span that of the maximumexpected image-displacement while its height should cover image-height,(2 mm×5 mm adequate here for spot diameter of 1.85 mm. Linearity may bekept to a fraction of 1% especially where operating range is compressed;also a "slit-image" is preferred to a relatively circular "spot". Also,a dual-axis lateral cell may be used where "height-displacement"information is also desired, as workers will appreciate.

Focal length of the system lens is chosen to meet available spacerestrictions and to result in a usable image of the IRED and sufficientimage shift for a strong signal from the detector. The 25 mm, f/2 lensused here focused an image of the IRED 1.85 mm in diameter and resultedin a lateral shift of ±1.31 mm when the galvo mirror was pivoted throughits ±1.5° required range. Since the detector active area was 2 mm highby 5 mm wide, the image fit nicely on the detector with adequate marginfor mechnaical error.

The mentioned "lateral cell" position detector is illustrated in FIG. 6,with a representative circuit showing in FIG. 7. Workers will recognizecell 31 as comprising a cathode k superposed on a junction region Jwhich in turn rests upon a "distributed resistance" zone R from whichanodes A, A' project. When light energy falls onto the active siliconarea (surface 31-5 of cathode k) of the lateral cell, electron-holepairs are created and a very small current flows through the resistivebackplane R of the cell to anodes A,A' (two ohmic contacts) located atopposite edges of the backplane. The differential current from these twocontacts is proportional to the location of the "optical centroid" ofthe light pattern falling on the active area and thus the angularposition of the galvo mirror. Since the outputs react to the centroid ofthe light, the image falling on the detector need not be sharply focusedor of uniform intensity. This reduces the accuracy burden on the opticalsystem.

FIG. 7 is a theoretical, electrical model of the lateral cell 31. Themagnitude of the current source is proportional to the total lightenergy falling on the detector. The position of the "potentiometerwiper" represents the position of the impinging light beam on thedetector. Bandwidth of the cell is determined by the internalcapacitance of the device and the resistance seen by that capacitance.(When the wiper shown is at center position, one model detector used, aSilicon Detector Corporation model SD-200-21-21-391--has a bandwidth of222 KHz--note capacitance c=160 pF, R₂ =18 K-ohm, plus "very large" R₁and wiper as "centered"--thus f=1/2R₁ c ).

FIG. 8 is a block diagram of the electrical signal processing system.Two transimpedance amplifiers A_(t), A'_(t) (not shown in FIGS. 2,3) arecontained with the detector 31 inside the mu-metal enclosure 2, mountedon the translator. They convert the (preamplified, differentiated)currents from the detector 31 to low impedance voltages suitable forcommunication through flexible shielded cables. These cables connect thetranslator to the signal processing circuit (board) sp, mounted on astationary machine part (preferably in OMM mainframe card cage asworkers will appreciate). The cable signals are received by twodifferential-input line receivers R, R'; and the voltage outputs fromthese amplifiers (each proportional to a respective current output froma respective port of the lateral cell) are applied to a sum/differencestage to produce mirror-feedback control signals (s_(g)). Thus, for agiven amount of light on detector 31, the difference in these voltagesindicates mirror position, (output signal s_(g)).

The LED tends to decrease in brightness as it ages and this would causea reduction in output scale factor. The "difference-divided-by-sum"circuit (A_(D), SA, AD) is included to compensate for this LED aging andalso to make the output immune to fluctuations in light intensity,(divide circuit comprising a multiplier used in the feedback loop of anopamp). This will minimize "scale factor" errors [such can also resultfrom variance in power, radiation intensity to cell 31].

Workers will understand that enclosure 2 and unit GU (FIG. 3) arepreferably mounted, as mentioned, to be translated forrough-track-positioning as known in the art. Thus, to minimize theinertia of the translator, the size and weight of the assembly are keptsmall, and only the circuitry necessary to preamplify the detectorsignals is carried in the mu-metal enclosure.

FIG. 9 is a schematic diagram of the portion of the circuit carried onthe translator. The preferred transimpedance amplifiers (e.g., typeLM101A op amps) are selected for their inherently-low offset voltages.To shield components from magnetic and electrical crosstalk from thegalvo coil, parts are located as close to the detector element aspossible and preferably behind it. Inputs to the op amps are extended tothe detector with twisted wire pairs in order to "common mode" theinduced currents. Bias balancing resistors are connected at thedetector's ground point so that this ground path appears identical tothe signal path from the detector. Outputs received similar treatment:Two twisted-pair, shielded cables are used with the "low" or referencelead in each cable connected to the same detector ground point and theshields connected to power supply ground. To further reduce crosstalk, alaminated mu-metal and copper shield are placed around the galvo andconnected to supply ground.

FIG. 10 shows the preferred signal processor circuitry sp. All op ampsare type LM101A with power supply decouplers and compensationcapacitors, giving each op amp a prescribed gain bandwidth product(e.g., 0.7 MHz). The differential line receivers have suitable gain(e.g., 12 dB) and bandwidth (e.g., 31 KHz) to minimize noise. A polarityreversal switch S72 is included for convenience when used with a controlloop.

The "difference-divided-by-sum" circuit consists of difference ampA_(D), summing amp SA, analog multiplier/op amp AD. The difference ampand "summer" are both straightforward as workers know. Analog divisionis realized by placing the multiplier (preferably #MC1494 byMotorola--see also "Motorola Linear I.C.", second printing 1978) in thefeedback path of the op amp. When this configuration is used, it isnecessary to prevent V_(y) from going negative which would result in apositive feedback condition. To accomplish this, a positive rectifiercircuit is combined in the summing amp SA.

Associated with the divider circuit are trimpots R85, R86 and R87 whichnull out DC offsets in the divider. R88 and R89 establish divider scalefactor; R89 may be adjusted to give the desired number of volts perdegree. Switch S71, when placed in the "CAL" position, provides a signalinjection point for divider calibration. Zener diodes are used toprotect the inputs of the analog mulitplier. Likewise, R80, C84, R81 andC85 are included to prevent oscillations.

The output of the divider section is the CURRENT ANGLE output whichsatisfies the machine objective. There are also two TTL outputs used forsystem diagnostics, as described below:

Type LM311 voltage comparators IC-76 and IC-77 and open collector NANDgates form a window detector with thresholds set at +15 mV and -15 mVand hysteresis set at 15 mV to eliminate chattering. When the galvomirror is at center position, the IC-77 output is low and the IC-76output is high (TTL levels), causing the GALVO CENTER output to go high.

The GALVO LIMIT output prompts the central control system that forwhatever reasons the total light falling on the detector is below aminimum acceptable value. This could be caused by excessive galvo mirrorangles or by an IRED failure. IC-75 is an LM311 voltage comparator whichuses as its input the output of summing amp SA.

If all tolerances were allowed to accumulate in the same direction, theoutput of the circuit would carry a certain mV offset. Fortunately, thiscan be nulled at a given ambient temperature by adjusting the opticalalignment. To minimize drift caused by temperature, the transimpedanceamplifiers were specially selected to have offset voltage drifts in thesame direction with temperature.

The accuracy and linearity of the system can be tested by substituting acalibrated angular tilt stage (see FIG. 11), for the reflector since thepivoting stage's mirror reflects the infra-red beam twice, every degreeof tilt corresponds to two degrees of pivot from the galvo. Circuitoutput (CURRENT ANGLE) may then be plotted against the angle of thesubstitute reflector.

Requirements of the servo system in which this feedback element was usedcalled for a gain at zero-crossing of 10-8-9volts per degree of mirrordeflection. The linearity test was performed at a reduced scale factorso that a larger mirror deflection could be tested without saturatingthe output op amps.

Linearity can be brought to within ±5% of the straight-line extensiontangent to the curve at zero crossing. However, quality diminishestoward the extremes of the angle range because of the distortion andlight loss in the optics.

System bandwidth may be tested two ways: (1) Drive the galvo coil with asinewave while monitoring CURRENT ANGLE output, or (2) modulate the IREDand measure the output from the summing amp, A71.

Method (1) has the advantage of being direct. However, the galvo hassecond-order response characteristic which must be accounted for (e.g.,notably a 40 dB per decade roll-off commencing at 50 Hz which attenuateshigh frequency signals to the noise floor before the bandwidth limit isreached).

Method (2) bypasses the galvo mirror and allows a bandwidth measurementof all stages except the difference amp and divider. Overall bandwidthis limited by the differential line receivers.

Modified embodiments, FIGS. 12, 13:

FIG. 12 illustrates an arrangement GUU essentially like that of FIG. 2(GU) except as otherwise indicated. Here, the galvo mirror g'_(m) ismodified so that its "back side" (opposite that optimized to reflectlaser image, e.g., at 633 nm) is provided with reflector means M_(ir)adapted to optimally reflect monitor radiation from a source LED (e.g.,IR-LED at 940 nm with built-in lens as known in the art). This (IR)illumination is to be focused by focus means fc and detected at detectorsD (e.g., preferably to split-cell (bi-cell) detector as known in theart). A light shielding enclosure ch' is used as before to house thecomponents. Preferably the monitor-optics is adapted to increase detectsignal (e.g., large lens; close to M_(ir)) and to decrease noise (e.g.,total shielding from laser image and other stray illumination), thusenhancing S/N.

Also, the electronics package es' (e.g., pre-amps) is preferablydisposed to be magnetic shielded behind detector sD, reducinginterference by flux from coils of galvo mirror g'_(m), this beingweakest along a path normal to coil-winding, of course).

FIG. 13 is similar except that a related source S and detector D aredisposed in relatively the same direction. Here, as in the arrangementof FIG. 12, use of a lens compact package and larger lenses can reduceany optical distortion that may occur (e.g., in the linearity of thesystem because the image of the IRED "vignettes" as the monitor-beamswings off-center).

Also, the arrangements of FIGS. 12, 13 have featured "segmented positionsensors" (sp 5) as opposed to the "lateral cell" aforedescribed, (eachbeing a form of silicon photodiode well known in the art) as workersknow.

Segmented position sensors of the quadrant and bi-cell variety exhibitthe greater position sensitivity and resolution but have dynamicoperating ranges which are limited to the dimensions of the opticalimage focused onto the detector. Segmented cells require uniformillumination intensity in the spot to achieve good linearity. They canoperate at bandwidths of well over 100 megahertz as may be required inpulsed and high-speed tracking applications.

One form of sps is a quadrant detector: a monolithic structure with fourdistinct separated active areas (anodes); and a cathode common to allfour regions.

The simplest use of a quadrant detector involves imaging a uniform spotof light onto the detector in such a way that the center of the detectoris included within the light spot. Photo-generated currents are therebyinduced in each of the four active regions and flow into the externalcircuit. The magnitude of the current flowing from each quadrant isproportional to the integrated light flux falling on that quadrant.Presuming a uniform light intensity, the difference between signals fromopposite quadrants, divided by the sum of currents from oppositequadrants, yields a normalized transfer function specifying the positionof the spot centroid as a fraction of the overall operating range.

The operating range for the above mode of operation is equal to theradius of the light spot. When the light spot is more than a radius awayfrom center, all four quadrants are no longer illuminated and thetransfer function no longer represents the analog spot position.

A single-axis cousin of the quadrant detector is the "bi-cell". Thissegmented position sensor has only two active areas and indicates theposition of a light spot with respect to the boundary between theseactive areas. Signal processing is performed in the same manner as forthe quadrant detector. An example of a bi-cell is Silicon DetectorCorporation's SD-113-24-21-021 which is housed in a TO-5 package. Theactive area containing both anodes is about 0.1×0.1 inches.

The linearity of a segmented detector would actually be quite good if alight spot of perfectly uniform intensity were used. Most practicallyrealizable light spots, however, have more of a gaussian intensitydistribution and may exhibit any number of other abberations.Consequently, segmented detectors often deviate greatly from the ideallinear transfer characteristic. Segmented detectors are used mostsuccessfully in nulling applications where a very sensitive measure ofsmall diviations about zero are required.

Exemplary Optical R/W system (FIGS. 14-20):

FIGS. 14-20 show a representative optical record/read system apt forusing the subject invention. This system will be seen to include a"galvo unit" like that described, this unit including a galvo mirrormounted on translator stage. Together these accommodate the necessaryradial beam scanning to correct for disk runout and to enable trackselection within the field of view of the lens. Disk runout correctionduty and track-seek functions are shared to optimize translator positionusing "track-follow" and "track-seek" operations. As mentioned, it isnecessary to communicate to the control system a feedback signalindicating the angular position of the galvo mirror. Such a controlsystem is also used to reduce the effects of cross-coupling betweenfocus motor and galvo.

The translator may be viewed as used mainly for "coarse track-seek"duty, especially since it is relatively heavy and slow-response (itcarries the galvo coils, etc., plus the focus means. Conversely, thelight galvo mirror can provide quick response (over a limited trackspan--e.g., about 30 tracks, each about 0.2 microns or 8×10^(-6") wide),typically, the galvo will probe "ahead" of the translator, withposition-error signals fed to the translator so it may "catch-up" afterthe galvo has located the "target track" as workers well know.

FIG. 14 generally illustrates the basic portions of a preferred opticalrecording and reading system apt for using the present invention. Thedata to be recorded is first applied to recording circuitry 10 whichencodes the applied data using, for example, a conventional encodingformat of the type employed for magnetic recording, such asnon-return-to-zero, return-to-zero, etc. Conventional error checking mayalso be provided for the encoded signal.

The encoded data 10a from the recording circuitry 10 is applied to alaser optical system 12. The laser optical system 12 generates threelaser beams 12a, 12b and 12c which are focused at spaced locations alongthe center line of the same selected track of a preformatted opticaldisk 15 supported on a precision spindle 16 for rotation by a motor 18.The optical disk 15 may, for example, be a trilayer disk of the typedisclosed in the aforementioned U.S. Pat. No. 4,222,071.

Laser beam 12a is a writing beam which is modulated by the encoded dataso as to form optically detectable changes in a selected track of theoptical disk 15 representative of the encoded data. It is to beunderstood that the optically detectable changes produced in the disk bythe write laser beam 12a need not be physical changes, such as pits orphysical holes. The only requirement is that optically detectablechanges be produced in selected areas of the disk in response to thewrite laser beam 12a which are representative of the encoded data 10a.For the purpose of this description, all of the possible types ofoptical changes that can be produced will hereinafter be referred to as"optical holes".

Laser beams 12b and 12c shown in FIG. 14 are reading beams. As typicallyillustrated in FIG. 15, the reading beam 12b is a read-after write beamwhich is accordingly focused behind the writing beam 12a on the centerline 17a of a selected track 17, while the reading beam 12b is aread-before-write beam and is accordingly focused ahead of the writingbeam 12a. The read beams are reflected from the disk 15 back to theoptical system 12 which, in response thereto, derives a plurality ofdetection signals 14a, 14b and 14c which are applied to signalprocessing electronics 20. The signal processing electronics 20 usesthese detected signals 14a, 14b and 14c to provide an output data signal20a corresponding to data read from the optical disk 15, along with thesignals 20b and 20c respectively identifying the track and sectorlocations on the disk from which the data is read.

The signal processing electronics 20 also produces control signals 21a,21b, 21c, 21d and 21e, 20f for use in providing precise control of diskrotational speed, beam focusing and track following. More specifically,control signal 21a is applied to the optical disk motor 18 to provideaccurate speed control during recording and reading; control signal 21bis applied to the laser optical system 12 for controlling the radialposition of the laser beams 12a, 12b and 12c for the purpose ofselecting a desired track; control signal 21c is applied to the laseroptical system 12 for providing precise track following of the laserbeams on the selected track; control signal 21d is applied to the laseroptical system 12 for providing the precise focusing of the laser beams12a, 12b and 12c; and control signal 21e is applied to the recordingcircuitry 10 for interrupting recording if the reflectedread-before-write beam indicates that the track ahead containspreviously recorded data.

Reference is next directed to FIG. 16 which illustrates a preferredversion of the laser optical system 12 generally shown in FIG. 14. Thevarious components of this laser optical system are illustrated in blockand schematic form in FIG. 16 since their implementation can readily beprovided by those skilled in the art, as will be evident from theaforementioned references.

As shown in FIG. 16, a laser 30 provides a beam 30a having a wavelengthof, for example, 0.633 um and a power level of, for example, 12 mW. Thislaser beam 30a is applied to a first beam splitter 32 which splits thebeam into a high power beam 32a and a low power beam 32b. The low powerbeam 32b is applied to a second beam splitter 34 which further splitsthe beam 32b to provide read-after-write and read-before-write 12b and12c, respectively. It is to be understood that a separate laser could beemployed for providing one or more of the above beams if so desired.

The high power beam 32a in FIG. 16 is applied to a high speedlight-modulator 36 which modulates the beam 32a in response to theencoded data 10a provided at the output from the recording circuitry 10in FIG. 14. This light-modulator 36 may, for example, be anacousto-optical digital modulator. The resulting modulated high powerbeam at the output of the modulator 36 is used as the write beam 12a ofthe system and is applied to a beam combiner and splitter 38 along withthe read beams 12b and 12c which combines the beams taking into accounttheir previously described spacing along the selected track of the disk15 as typically illustrated in FIG. 15. The resulting three laser beams12a, 12b and 12c are then reflected off of a mirror 40 mounted to agalvanometer 42. The galvanometer 42 is responsive to the control signal20d from the signal processing electronics 20 (FIG. 14) so as to causethe mirror 40 to be appropriately deflected as necessary to provide forprecise following along the center line of the selected track.

After reflection from the mirror 40, the laser beams 12a, 12b and 12care then directed to an objective lens assembly 44 mounted on a focusingmotor 46. The motor 46 operates in response to the control signal 20dfrom the signal processing electronics 20 in FIG. 14 to move theobjective lens assembly 44 towards and away from the disk 15 so as tothereby maintain accurate focusing of the beams 12a, 12b and 12c on aselected track of the disk. Track selection is provided by controllingthe radial position of the beams 12a, 12b and 12c relative to the disk.This is accomplished using a linear motor 48 coupled to the objectivelens assembly 44 and responsive to the control signal 20d from thesignal processing electronics 20 in FIG. 14.

It will be understood that the two read beams 12b and 12c shown in FIG.16 are reflected from the disk 15 with a reflected power which ismodulated in accordance with the recorded pattern over which the beamspass. The reflected read beams 12b and 12c pass back to the beamcombiner and splitter 38 via the objective lens assembly 44 and themirror 40. The beam combiner and splitter 38 directs the reflected beamsto optical detection circuitry 49 which converts the beams intocorresponding read-after-write and read-before-write analog electricalsignals 14a and 14b which are applied to the signal processingelectronics 20 as shown in FIG. 14. Also, at least one of the reflectedread beams 12a and 12b is applied to an optical focus detector 47 whichprovides an electrical signal 14c to the signal processing electronics20 which is indicative of the quality of focusing of the beams on theselected track.

Next to be considered is the manner in which preformatting is providedfor the optical disk 15 in FIG. 14 in accordance with this system. Anexample of a typical formatting arrangement is illustrated in FIGS. 16and 17.

As generally indicated in FIG. 17, the optical disk 15 in the preferredembodiment being described contains a large plurality of circumferentialtracks 17. The disk 15 is also divided into a plurality of sectors 19.As indicated in FIG. 17, each track 17 within a sector 19 comprises aheader 51 and a data recording portion 52. The data recording portion 52is the portion into which data is written during recording and comprisesthe greater portion of the track length within each sector 19. Theheader 51 of a track 17 is encountered first in each sector 19 and isprovided on the disk prior to recording. The provision of such headers51 on a disk prior to data recording is typically referred to as"formatting" the disk, and the resulting disk is considered to be"preformatted".

FIG. 18 illustrates an example of a preformatted header 51 provided inaccordance with this system for each track 17 in each sector 19 of thedisk 15 of FIG. 17. Although the optical holes constituting the header51 need not be physically obserable, as mentioned previously, it will beassumed as an example that physical holes, such as pits, are employedfor the exemplary header shown in FIG. 18. It will also be assumed thata pit exhibits a relatively high reflectance to an incident beam whileunrecorded disk areas exhibit a relatively low reflectance. It is to beunderstood that an arrangement may be employed in which a portion of theoptical recording, such as the header, is recorded using physical holes,such as pits, and the remaining recorded portions, such as thosecontaining data, are recorded using optical holes. It is additionally tobe understood that special purpose recording apparatus may be used forproviding headers on a disk (that is, preformatting the disk) whichapparatus is different from that used for recording data.

Before continuing with the description of the header shown in FIG. 18,reference is first directed to FIG. 20 which illustrates a cross-sectionof a disk 15 which may be employed in accordance with the invention. Asupporting substrate 90 such as a 0.1 to 0.3 inch thick disk of aluminumis coated with an organic smoothing layer 92 of, for example, 20-60microns prior to deposition thereon of a highly reflective opaque layer94 of aluminum which may, for example, have a thickness of 400-800Angstroms. An inorganic dielectric layer 96 such as a 800-1200 Angstromlayer of silicon dioxide which is transparent at the laser frequency isdeposited over the aluminum reflector layer 94. An absorbing layer 98which is absorptive at the laser frequency is then deposited over thedielectric layer 96. This absorbing layer 98 may for example be a 50 to300 Angstrom layer of a metal such as tellurium. Finally, the absorbinglayer 98 is overcoated with a protective layer 100, such as a siliconresin having a thickness of, for example, 150 to 500 Angstroms.

Still with reference to FIG. 20, an anti-reflection (dark mirror)condition for a laser beam incident on the disk 15 is produced byappropriately choosing the thicknesses and optical characteristics ofthe layers 94, 96 and 98. Recording on such a disk 15 as illustrated inFIG. 20 is then accomplished by employing an appropriately focused,intensity-modulated recording laser beam (such as laser beam 12a inFIGS. 14-16) which records information by forming pits 98a in theabsorbing layer 98 along a selected track, and spacing and dimensions ofthe pits 98a being representative of the recorded data. Information isread from the disk 15 using an appropriately focused reading laser beam(such as laser beams 12b and 12c in FIGS. 14-16) which is chosen to beof insufficient intensity to affect undisturbed regions 98b of theabsorbing layer 98 and has a frequency at which these undisturbedregions 100 exhibit the previously mentioned anti-reflection condition.As a result, the reflected reading beam will be intensity modulated bythe pits 98a since the reading beam will experience a relatively highreflection when the beam is incident on a pit 98a, and a relatively lowreflection when the reading beam is incident on an undisturbed region98b. It will be understood that dust particles on the upper surface ofthe protective layer 100 will be far removed from the focal plane of theoptical system so as to have a negligible effect on the above describedrecording and reading operations.

Reference is now directed back to FIG. 18 for a more detailedconsideration of the header 51. Since the header 51 is used inconjunction with the signal processing electronics 20 in FIG. 14 toprovide for reliable and precise operation of the system, it will behelpful to describe the construction and arrangement of the exemplaryheader 51 shown in FIG. 18 in conjunction with FIG. 19 which illustratesa preferred implementation of the signal processing electronics 20generally shown in FIG. 14. The individual components of FIG. 19 canreadily be implemented by those skilled in the art and are thus shown inblock form.

Referring to the preformatted heading 51 shown in FIG. 18, it will beseen that immediately following the left sector boundary 19a is arelatively large pit 54 providing a relatively large change in opticalreflectance which is used to provide synchronized timing for the signalprocessing electronics 20. This is accomplished by applying the detectedread-after-write signal 14a in FIG. 16 to a peak detector 73 via apreamplifier 71. The peak detector 73 outputs a narrow pulse 73acorresponding to the pit 54 which it recognizes as the largest peak inthe playback signal. This narrow output pulse 73a produced by the peakdetector 73 is then applied as a timing reference to conventional timingcircuitry 75 which generates various timing signals 10b, 21a, 75a, 75b,75c, 75d and 75e for synchronizing the operation of the system with thedisk 15. The purposes of these timing signals will become evident as thedescription proceeds.

Following pit 54 in FIG. 18 are two pits 56 and 58 elongated in adirection parallel to the track 17 and disposed on opposite sides of thetrack center line 17a in a staggered relationship. These pits 56 and 58are used to provide precise track following. This is accomplished inFIG. 19 by applying the amplified read-after-write beam provided at theoutput of the preamplifier 71 to up-down integrator circuitry 77. Theup-down integrator circuitry 77 integrates up in response to thedetected signal obtained when the read-after-write beam traverses theportion of the track 17 corresponding to the elongated pit 56, andintegrates down in response to the signal obtained when theread-after-write beam traverses the portion of the track 17corresponding to the elongated pit 58. It will be understood that thedifference between these two integrations will be a measure of thepreciseness of track following by the laser beams. The dimensions andlocations of the elongated pits 56 and 58 are chosen in conjunction withthe size of the focused beam so that even very small deviations of thebeam from the track center line 17a can be detected. This differenceprovided by integrator circuitry 77 when pits 56 and 58 are traversedduring each sector is accordingly used to produce the control signal 21cwhich is applied to the galvanometer 42 (FIG. 16) to provide for precisefollowing of a selected track by the laser beams.

It will be noted in FIG. 19 that the timing circuitry 75 provides timingsignals 75a and 75b to the up-down integrator circuitry 77. The timingsignal 75a is used to delineate the particular times during traversal ofthe header 51 of each sector for which up and down integrations shouldbe performed so as to appropriately correspond with the locations of theelongated pits 56 and 58. The timing signal 75b is provided during eachsector to the up-down integrator circuitry 77 to serve as a hold signalto hold until the next sector the resultant integrated value obtainedafter the read-after-write beam has completed traversing the secondelongated pit 58.

Following the elongated pits 56 and 58 in the exemplary header 51 shownin FIG. 18 are a plurality of pits 60 elongated perpendicularly to thetrack center line 17a. The locations and dimensions of the pits 60 arechosen so that the reflected signal obtained upon traversing these pits60 will have a peak value dependent upon the quality of focusing of theincident beam. This may be achieved, for example, by choosing thethickness of each pit 60 so that it is equal to the diameter of aproperly focused beam. Then, if the incident beam is larger than thethickness of a pit 60 because of improper focusing, the reflected beamwill have reduced power when each pit 60 is traversed, since only aportion of the beam will be reflected. It will also be understood thatthe spacing between the pits 60 determines the frequency at which thereflected beam is modulated when traversing the pits 60.

Referring again to FIG. 19, it will be understood that theread-after-write beam 14a applied to the preamplifier 71 during theperiod that the focusing pits 60 are being traversed contains theresulting focusing information. Accordingly, a peak detector 64, whichis enabled by timing signal 75c during the period that theread-after-write beam is traversing the focusing pits 60, is provided toreceive the amplified read-after-write beam at the output of thepreamplifier 71. The peak detector 64 is adapted to respond to themagnitude of the applied signal within a frequency range determined bythe spacing of the pits 60 to produce an output signal 64a which is ameasure of the quality of focusing.

The output signal 64a from the peak detector 64 is applied to a signaladded 66 along with the signal 14c provided by the optical focusdetector 47 in FIG. 16. The signal adder 66 appropriately combines thesetwo signals 14c and 64a to produce the resulting signal 21d shown inFIG. 14 which is applied to the focusing motor 46 for maintainingprecise focusing of the incident laser beams on the disk.

The purpose of providing a signal 21d to the focusing motor 46 comprisedof the signal 14c from the peak optical focus detector 47 and the signal64a from the peak detector 64 will now be further considered. It will beunderstood that, for the disk rotational speeds and recording densitycontemplated for the preferred system being described, the optical focusdetector 47 in FIG. 16 will have a relatively slow response time andwill accordingly provide only a coarse control of the focusing distanceas the disk rotates. In accordance with the present system, asexemplified here, significantly more precision and reliability areachieved by providing the additional focusing capability made possibleusing the focusing pits 60 in each header 51 as shown in FIG. 18. Asjust described, such focusing pits 60 permit deriving a peak-detectedsignal 64a which will have a significantly greater response time thancan be provided by the signal 14c from the optical focus detector 47,thereby providing fast-acting control of focusing for each sector 19 bythe focus motor 46 which is designed to provide the appropriate fastresponse. Of course, as also applies to the track following pits 56 and58, the header 51 is repeated a sufficient number of times around eachcircumferential track 17 to obtain the desired precise and fast-actingcontrol of focusing as well as of track following.

Continuing with the description of the header 51 shown in FIG. 18, theabove described focusing pits 60 are followed by pits 72 recorded so asto provide an identification of the particular track and sector beingtraversed by the laser beams. In other words, the pits 72 represent atrack and sector address and conventional encoding can be employed forthis purpose, such as is used for identifying tracks and sectors onmagnetic disks.

As shown in FIG. 19, the amplified read-after-write signal at the outputof the preamplifier 71 is applied along with an appropriate enablingtiming signal 75d to a track and sector decode 78 which provides trackand sector signals 20a and 20b respectively (see also FIG. 14)indicating the track and sector being traversed by the laser beams. Thetrack signal 20b is also applied to track selection circuitry 80 alongwith a track command signal 80a indicative of a selected track to whichit is desired that the laser beams be positioned. The track selectioncircuitry 80 compares the track indicated by the track signal 20b withthe track requested by the track command signal 80a and in responsethereto produces the signal 21b which is applied to the linear motor 48in FIG. 16 to center the laser beams over the selected track.

Referring to FIG. 17 along with FIG. 18, it will be understood that, inthe exemplary header 51 illustrated, the pits 72 which provide track andsector address identification are the last portion of the header 51. Aspointed out previously, the resulting disk containing these headers isconsidered to be preformatted. Such preformatted disks will typically beprovided to users who will make use of the preformatted headers inconjunction with signal processing electronics 20 such as illustrated inFIG. 19 for recording and reading data in the data recording portion 52of each track 17 in each sector 19.

The amplified read-after-write signal provided at the output of thepreamplifier 71 in FIG. 19 is also used during the reading of data fromthe data recording portion 51 of each sector 19 (FIGS. 17 and 18).Accordingly, the implementation of FIG. 19 includes data read circuitry82 to which the output of the preamplifier 71 is applied for providing adata output signal 20a (see also FIG. 14) corresponding to the recordeddigital data. The data read circuitry 82 is enabled during the timeperiod that the read-after-write beam is traversing the data portion 52of each sector 19 by the timing signal 75e. The resulting data outputsignal 20a is applied to an appropriate utilization device (not shown)along with the track and sector signals 20b and 20c which identify thetrack and sector from which data is read. This reading operation is alsoperformed during data recording to check that data is accurately beingrecorded.

An additional feature of the present system involves the manner in whichthe read-before-write beam 12c (FIG. 15) is employed. It will beappreciated that the density of recording made possible by the presentinvention is quite high. Thus, the possibility exists that an error inpositioning of the laser beams may arise during recording which couldresult in destroying previously recorded data. Such previously recordeddata can be very expensive to replace, if not lost forever if there isno back-up. This problem is prevented by the present system by makinguse of the read-before-write beam 12c.

As illustrated in FIG. 19, the read-before-write signal 14b obtainedfrom the optical detection circuitry 49 in FIG. 16, is applied to apreamplifier 91 whose output 91a is in turn applied to a data detector95 via filter circuitry 93. The filter circuitry 93 is provided toprevent noise from interfering with the operation of the data detector95. The data detector 95 is responsive to the presence of recorded datato produce an interrupt signal 95a which is applied to the recordingcircuitry 10 (FIG. 14) to halt recording, thereby protecting previouslyrecorded data. It is to be understood that the read-before-write beammay also be used for other purposes, such as to check the quality of thetrack prior to recording, or to provide more precise track followingand/or focusing control.

Conclusion:

A radiant energy beam scanner/position sensor has been shown to operateusing a solid state position detect means (e.g., a lateral cell of asegmented cell) together with fairly common active electroniccomponents. Workers will recognize that one may modify certain aspects.For instance, in a circuit like the embodiment, one may minimize theeffects of DC offset in the op amps by using a modulated infra-red beamsystem. IRED intensity could be modulated at a carrier frequency withinthe bandwidth of the detector (i.e., 75 KHz). Thus, the signals at thedetector outputs would constitute this carrier, amplitude-modulateddifferentially by mirror position. Synchronous demodulation could bedone by a balanced demodulator or by active circuitry as known in theart.

Also, the optical system can be optimized. Larger lenses and a moresuitable monitor-beam reflector surface will improve the signal leveland linearity. A stronger signal will require less gain, and this canreduce offset and crosstalk, while improving S/N. Improved performancecould be realized by increasing the focal length and aperture of thelens. A larger aperture will collect more radiation from the LED (to besent to the detector). A longer focal length will enable a greaterportion of the detector-surface to be used, (e.g., the focal lengthcould be increased in a like embodiment to about 49 mm). And, by usingmore detector area one can reduce the amplification required and improvesignal-to-noise, while reducing both crosstalk and offsets (e.g., a 34mm lens of 17 mm diameter can yield 9 dB improvement in a likeembodiment).

The mechanical layout could be rearranged as shown in FIG. 12. Thisasymmetrical design allows the first stage amplifiers to be placedcloser to the detector and farther from the source of crosstalk. Aslightly larger enclosure (ch) and a redesign of the mounting hardwareis required for this, of course.

It is advisable to design the mechanical layout so that one may adjustthe position of the galvo and the reflector in the course of installingthe assembly on the translator. This greatly facilitates opticalalignment.

If it is necessary to have a DC level that is very precise (e.g., tominimize offset), then the system can be "shifted-up" in frequencydomain. The LED brightness could be modulated at a carrier frequency andthe lateral cell outputs synchronously detected. A high carrierfrequency will also make the detector circuit relatively insensitive tolow-frequency noise (e.g., 120 Hz interference). However, such a schemewill probably dictate that a "higher bandwidth" detector, like thementioned "segmented element" type.

Since system performance can be impaired by poor infra-red reflectanceof the galvo mirror surface, it will usually be preferable to use theback surface of the galvo mirror as the IR reflector (as in FIGS. 12,13). Then, folding the optics is unnecessary and the front surface ofmirror g_(m) can be kept optimized for laser illumination.

Workers will appreciate how aptly such a radiant-energy position monitormeans may be used to control and track the angular orientation ofmirrors like these used in optical disk drive apparatus as described. Inparticular it will be appreciated that such monitor units can be used toimprove the efficiency and cost-effectiveness of arrangements using sucha mirror and to increase their operating speed--something workers in theart will applaud. Workers will also appreciate that such monitors may beused to track and control other similar movable reflectors in relatedenvironments.

It will be understood that the preferred embodiments described hereinare only exemplary, and that the invention is capable of manymodifications and variations in construction, arrangement and usewithout departing from the spirit of the invention.

Further modifications of the invention are also possible. For example,the means and methods disclosed herein are also applicable topositioning other radiation director means and related reflectors insimilar systems and environments. For instance, related embodiments maybe employed to position reflectors used with other forms ofrecording/reproducing systems using different radiant energy--e.g.,those in which data is recorded and reproduced holographically.

The above examples of possible variations of the present invention aremerely illustrative. Accordingly, the present invention is to beconsidered as including all possible modifications and variations comingwithin the scope of the invention as defined by the appended claims.

What is claimed is:
 1. In an optical data disk system where a radiantenergy information laser beam of prescribed first wavelength is to beselectively positioned by rotatable scan-mirror means, the mirror meansbeing arranged to provide a laser-reflecting mirror surface highlyreflective of said laser information beam, this laser beam beingcomprised of:a "write beam" and two "read beams", all adapted to beprojected on a given, selected track of a rotating optical recordingdisk with a first read beam adapted for "read-before-write" function andthe other read beam adapted for "read-after-write" function; an improvedmethod of providing rotation monitor means to automatically monitor theangular orientation of said mirror means, and provide an output signalreflecting this, this method comprising: operatively associatingreflector means with said mirror means to be rotated conjunctivelytherewith; arranging related source means to comprise an LED sourceadapted to an IR monitor beam of prescribed second wavelength, and toproject this beam to be reflected by said reflector means; providingrelated detector means which includes active extended surface meanswhich is adapted and disposed to receive said monitor beam whereby theangular rotation of said mirror means is converted to a prescribedlinear displacement across this surface means so as to generate positionsignals representing the relative position of the monitor beam on thesurface means; arranging and disposing optical means intermediate saidsource means and said detector means so as to convert the angularrotation of said mirror means to linear displacement along said detectorsurface means; and cooperatively arranging related utilization meansadapted to process said position signals and generate said output signalfor use in positioning said information beam; the utilization meansincluding amplifying/processing circuit means disposed to beelectro-magnetically shielded by said detect means and/or said sourcemeans; while also adapting said mirror means to additionally includerelated monitor-reflector means disposed opposite thereto and adapted toreflect said monitor beam from said source means to said detector meansso as to represent the angular rotation of said mirror surface duringscanning of said laser beam as prescribed linear displacement on saiddetector surface means.
 2. In an optical data disk system where aradiant energy information laser beam of prescribed first wavelength isto be selectively positioned by rotatable scan-mirror means, the mirrormeans being arranged to provide a laser-reflecting mirror surface highlyreflective of said laser information beam,an improved method ofproviding rotation monitor means to automatically monitor the angularorientation of said mirror means, and provide an output signalreflecting this, this method comprising: operatively associatingreflector means with said mirror means to be rotated conjunctivelytherewith; arranging related source means to provide a radiant energymonitor beam of prescribed second wavelength, and to project this beamto be reflected by said reflector means; providing related detectormeans which includes active extended surface means which is adapted anddisposed to receive said monitor beam whereby the angular rotation ofsaid mirror means is converted to a prescribed linear displacementacross this surface means so as to generate position signalsrepresenting the relative position of the monitor beam on the surfacemeans; said source and detector means being disposed and packaged to beoptically shielded from said information beam; and cooperativelyarranging related utilization means adapted to process said positionsignals and generate said output signal for use in positioning saidinformation beam; the utilization means including amplifying/processingcircuit means disposed to be electro-magnetically shielded by saiddetect means and/or said source means; while also adapting said mirrormeans to additionally include related monitor-reflector means disposedopposite thereto and adapted to reflect said monitor beam from saidsource means to said detector means so as to represent the angularrotation of said mirror surface during scanning of said laser beam asprescribed linear displacement on said detector surface means.